The present invention relates to a machine for producing a fiber web, particularly a paper web, and particularly relates to the dry end of the machine. The machine has a press section followed in the web path by a dryer section. The dryer section comprises a plurality of separated dryer groups, each operable at a respective different speed. Each dryer group includes a plurality of dryers, a plurality of web path reversal and suction rolls, one between each two dryers, and an endless loop web support belt, which is sometimes a dryer felt, passing around the dryer group in a meander path past the dryer cylinders and the reversal rolls and past guide means which guide the support belt in the endless loop. After the web to be produced from a fiber suspension is formed and partly dewatered in a wire end or forming end or wet end of the machine, the web is dewatered mechanically as far as possible in a press. Then the web is dried in a dry end comprised of heatable drying cylinders. A dry end suitable for this purpose forms the object of German Patent Application P 41 42 524.3, which is equivalent to U.S. application No. 07/844,145, filed Mar. 21, 1992 now U.S. Pat. No. 5,241,761 and several U.S. Patents.
One performance requirement for such a paper manufacturing machine is its suitability for extremely high operating speeds, on an order of magnitude of 1000 to 2000 m/min. Despite this high operating speed, the web should travel through the machine with the greatest possible safety, i.e. so that as few web tears as possible result. In other words, the travel efficiency or runability should be as high as possible.
In many cases, there is another requirement, namely drying the paper web to have an extremely low residual moisture content, e.g. of about 2 %. In these cases, drying is substantially more intense than for other types or uses of paper webs in which it is sufficient to obtain a higher residual moisture content in the web, of about 4 to 8 %. The extremely small residual moisture content of about 2% is necessary for producing certain types of paper, such as for the further processing of papers in a coating plant or in a calender. However, the decreased moisture content increases the danger that the paper web will tear, since the paper becomes brittle due to its extreme dryness and/or because the paper shrinks to a great extent, particularly in its longitudinal direction. Such shrinkage produces a quite high longitudinal tension in the web of paper.
When it is herein described that reversing suction rolls lie above or below neighboring dryer cylinders, that means that possibly the entire roll or only part of the roll is above or below the cylinder. However, at least the axes or centers of the rolls are above or below the axes of the dryer cylinders, as described. Some suction rolls can be so small and their axes can be so placed that the entire body of the roller is not beyond the radius of the adjacent dryer cylinders even though the center of the roller is above or below the centers of the adjacent dryer cylinders.
In order to increase the runability in known dryer sections, like that in U.S. Pat. No. 5,241,761, one proposal is now described. In as many dryer groups as possible, at least in the initial, or upstream or wetter region of the dry end, only the lower side of the web comes into contact with the drying cylinders. In other words, in the largest possible number of dryer groups, the drying cylinders all lie above the neighboring reversing suction rolls with which the dryer cylinders alternate along the web path. Only the next to the last dryer group, for instance, has a reverse arrangement in which the drying cylinders lie below the reversing suction rolls so that the top side of the paper web comes into direct contact with the drying cylinders of that group. Accordingly, within the initial region of the dry end, for instance between each two of the first four dryer groups, there are only so called "simple" places of separation between the adjacent dryer groups. This means that the web support belt of the next following succeeding dryer group contacts the last drying cylinder of the preceding dryer group at a place where the web of paper is no longer covered by the web support belt of the preceding dryer group. Such a known development of the place of separation is advantageous in two respects. The threading of the web of paper, for instance upon the starting of the paper machine operation or after a tear of the paper web, takes place completely automatically, without rope guidance being necessary, as is required in older arrangements. The web of paper travels just as reliably during the normal operation of the dry end from each preceding dryer group to the following dryer group. In an exceptional case and despite the favorable manner of construction described above, if a tear should take place in the web of paper, then the reject paper or broke moves readily downward from all drying cylinders of the dryer groups into the basement provided below the dry end.
In order to increase the runability, it is known to keep the web as reliably as possible on the web support belt at the place where the web runs off from each individual drying cylinder, and on the straight travel path from the drying cylinder to the following reversing suction roll. In this respect, the initial region of the dry end presents a particular problem because the paper web is still relatively wet there and it has a tendency to adhere to the wall of the drying cylinder and to detach itself temporarily from the support belt as the web leaves each dryer cylinder. In other words, a so called bubble is formed here between the web of paper and the support belt. In order to reduce the danger of the web of paper tearing, it is attempted to keep said bubble as small as possible. For this purpose, it is known to form a vacuum zone at the run-off place, shown in U.S. Pat. No. 4,359,828, FIG. 3. Another known measure consists of reducing the distance between the drying cylinder and the following reversing suction roll as much as possible, shown in International Application WO 83/00514, FIG. 2, or U.S. Pat. No. 4,905,379, FIG. 1.